In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise an imageable layer applied over the surface of a hydrophilic substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
To obtain a printing plate with imagewise distribution of printable regions, it is necessary to remove regions of an imaged imageable element. The most common method for removing the undesired regions is to contact the imaged element with a developer. If after exposure to radiation the exposed regions of the layer are removed by the developer revealing the underlying hydrophilic surface of the substrate, the element is a positive-working printing element. Conversely, if the unexposed regions are removed, the element is a negative-working element. In each instance, the regions of the imageable layer (i.e., the image areas) that remain after development are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Many alkaline-developable positive-working imageable elements comprise an imageable layer comprising a phenolic resin, such as a novolac resin, on a hydrophilic substrate, typically a specially treated aluminum sheet. For example, in one type of element the light sensitive layer comprises a novolac resin and a o-diazoquinone or diazonaphthoquinone, such as a naphthoquinonediazide sulfonic acid ester of a novolac resin. Upon exposure to ultraviolet light, the diazonaphthoquinone is converted to the corresponding carboxylic acid. The developer penetrates and removes the exposed regions of the imageable layer, revealing the underlying hydrophilic surface of the substrate, without substantially affecting the complimentary unexposed regions. During the printing process, the unexposed regions act as the image regions and accept ink.
The imageable layers of certain negative-working printing plates comprise a novolac resin, a cross-linking agent, and a radiation-sensitive component that produces acid on exposure. Subsequent heating cures the exposed regions, so that only the unexposed areas can be removed by an alkaline developer. The exposed regions, which remain after development, are oleophilic and will accept ink.
During the development process, the developer becomes loaded with components of the imageable layer that have been removed during development. Although some of the partly loaded developer is removed from the developing bath with the developed printing plates (drag-out) and is replaced with a replenisher, the amount of material in the developer increases as more printing plate precursors are developed. Once the developer becomes fully loaded, scum will form on the developed printing plates and on the components of the processor. The developer then must be replaced with fresh developer and the loaded developer disposed of.
Because of environmental considerations, the loaded developer must be treated before disposal to render it non-hazardous. If the loaded developer could be reused, it would be unnecessary to dispose of loaded developer and to prepare fresh developer. Thus, a need exists for a method for reusing loaded developer.